1. Field of the Invention
The present invention relates to a fuel cell formed by stacking a plurality of power generation cells. Each of the power generation cells includes an electrolyte electrode assembly and first and second separators sandwiching the membrane electrode assembly. The electrolyte electrode assembly includes a first electrode and a second electrode, and an electrolyte membrane interposed between the electrodes. The surface area of the second electrode is larger than the surface area of the first electrode.
2. Description of the Related Art
For example, a solid polymer fuel cell employs a membrane electrode assembly (MEA) which includes two electrodes (anode and cathode), and an electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a unit of a power generation cell for generating electricity. A predetermined number of the power generation cells are stacked together to form a stack of the fuel cell.
In the power generation cell, a fuel gas such as a gas chiefly containing hydrogen (hydrogen-containing gas) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. A gas chiefly containing oxygen (oxygen-containing gas) or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
Various sealing structures are used for preventing the leakage of the fuel gas and the oxygen-containing gas in the power generation cell. For example, Japanese laid-open patent publication 2002-25587 discloses a fuel cell which is designed to improve sealing characteristics between a membrane electrode assembly and separators. As shown in FIG. 9, the fuel cell includes a power generation cell formed by a membrane electrode assembly 1a interposed between first and second separators 2a, 3a. The membrane electrode assembly 1a includes an anode 5a, a cathode 6a, and a solid polymer electrolyte membrane 4a interposed between the anode 5a and the cathode 6a. The surface area of the anode 5a is larger than the surface area of the cathode 6a. 
A first seal 8a is attached to an inner surface of a second separator 3a. The first seal 8a is provided around the cathode 6a, and tightly in contact with the solid polymer electrolyte membrane 4a. Further, a second seal 8b is provided between the first and second separators 2a, 3a around the first seal 8a. 
The first and second seals 8a, 8b prevent the leakage of the reactant gases and the coolant. Further, the load balance in the surface of the power generation cell should be uniform, and the pressure load balance should not change depending on the power generation cell in order to achieve the uniform, and the stable power generation performance in each of the power generation cells. In particular, the pressure applied to the power generation surface should be kept at the desired level to stabilize the power generation performance. Further, each of the power generation cells should have a uniform space in the flow field so that the cross sectional area of the flow field does not change depending on the power generation cell, and the uniform flow rates of the reactant gases distributed from the reactant gas passages and the coolant distributed from the coolant passage can be achieved.
When thin metal separators are used as the first and second separators 2a, 3a, the balance of the line pressure (load) is not uniform, and the separators 2a, 3a tend to be deformed in the stacking direction. Thus, the pressure is applied to the sealing surface or the power generation surface excessively or insufficiently. As a result, it is difficult to achieve the desired power generation performance with the simple structure.
In the membrane electrode assembly 1a, the surface area of the anode 5a is larger than the surface area of the cathode 6a. In the structure, it is likely that the membrane electrode assembly 1a is deformed easily due to the difference between the surface pressure applied to the seal of the first separator 2a and the surface pressure applied to the seal of the second separator 3a. 